Galvanic cells having an aluminum anode are highly desirable as an energy source because of their high theoretical energy densities. Aluminum in aqueous solution has a standard oxidation potential of +1.7 v with 3 electrons transferred. If the aluminum electrode forms a couple with either aluminum hydroxide or aluminate ion in basic solution, the electrode couple has an oxidation potential of +2.3 v with 3 electrons transferred. When these electrochemical properties are considered along with the low density and small atomic weight, a theoretical energy density of 24.7 kJ/gram for the aluminum anode make it among the highest energy density anode materials. However, in alkaline aqueous electrolytes, two problems reduce aluminum anode performance. Side reactions compete with the electrochemical reactions and overvoltage effects reduce the cell energy output well below the theoretical value. As a result the anode efficiency is much lower than expected.
A variety of cathode reactions have been coupled with an aluminum anode in an aqueous alkaline electrolyte to form high-energy galvanic cells. In each case presented in the prior art galvanic designs, the lack of an efficient cathode reaction at a sufficient cathode potential is the major obstacle to development of high performance aluminum anode galvanic cells. A variety of aluminum anode galvanic cell systems are known in the art, including Al/Air (O2), Al/H2O2, Al/MnO2, Al/AgO, Al/S, Al/K3Fe(CN)6, and Al/NiOOH. All of the cathode reactions for these galvanic cells produce hydroxide ion to react with the aluminum to form either Al(OH)3(s) or Al(OH)4(aq)−1 as products. As an example, the reactions for the system, Al/H2O2, in an alkaline electrolyte are represented by four competing reactions, electrochemical, corrosion, direct, and decomposition as shown below:Electrochemical: 2Al(s)+3H2O2(aq)+2OH(aq)1→2Al(OH)4(aq)−1 Corrosion: 2Al(s)+6H2O→2Al(OH)3(s)+3H2(g) Direct: 2Al(s)+3H2O2(aq)→2Al(OH)3(s) Decomposition: 2H2O2(aq)→O2(g)+2H2O(l) 
Similar competing reactions are present to some extent in the other galvanic cell systems listed above. As such, alkali aluminum anode galvanic cell systems depend upon the cathode reaction to produce hydroxide ion in order to release electrical energy. Competing reactions become a problem when the hydroxide ion concentration is out of balance with the rest of the galvanic cell.
Balance of the cathode reaction based upon the reduction of hydrogen peroxide to produce hydroxide ions with the anode reaction of aluminum oxidation is the issue that leads to optimum energy output from the galvanic cell. Since hydrogen peroxide is a very weak acid, it is initially present in the electrolyte as the undissociated molecule. In the presence of hydroxide ion the hydrogen peroxide is hydrolyzed to the hydroperoxide ion. Catalytic decomposition of the undissociated hydrogen peroxide to release oxygen at the electrode competes with the combined hydrolysis and electron transfer to form hydroxide ion. The cathode reactions can be summarized as follows:H2O2(aq)+H2O(l)→O2H(aq)−1+H3O(aq)+1 H3O(aq)+1+OH(aq)1→2H2O(l) O2H(aq)−1+H2O(l)+2e→3OH(aq)1Eo=+0.87v overallH2O2(aq)+OH(aq)1+2e→3OH(aq)1 
Hydrogen peroxide ion requires the initial presence of hydroxide ion in the cathode compartment in order to prevent or restrict the catalyzed decomposition of peroxide and the release of oxygen gas.
The release of hydrogen from the aluminum corrosion reaction and the release of oxygen from the hydrogen peroxide decomposition reaction are problems in galvanic cells with an aluminum anode and hydrogen peroxide cathode reaction because pockets of gas interfere with the electrode processes. Aluminum hydroxide scale forms around the anode as the alkali hydroxide is depleted. Even if the hydrogen evolution is controlled, the formation of a solid aluminum hydroxide coating on the aluminum anode interferes with the anode reaction. Loss of aluminum and oxygen by means of the side reactions inherent in the prior art galvanic cells represents serious losses in cell efficiency and power delivery.
Additionally, several aluminum reactions can occur at the anode that depend upon the conditions in the electrochemical cell. Hydroxide ion levels at the anode influence the selectivity to form two aluminum products as shown below:Al(s)+4OH(aq)13e→Al(OH)4(aq)−1Eo=+2.3vAl(s)+3OH(aq)13e→Al(OH)3(s)Eo=+2.3v
The resultant product in the electrochemical reaction depends upon the levels of hydroxide ion available at the anode. Conditions for the optimum performance of the resultant galvanic cell favor the formation of the soluble aluminate ion. Deposition of the aluminum hydroxide is a problem when hydroxide levels are not adequate because the coating hampers the interaction of the aluminum electrode with the electrolyte solution.
Several techniques in the prior art have been used to avoid or reduce the impact of the side reactions and improve cell performance. Special alloys of aluminum containing, magnesium, tin, or gallium have been used to improve the reaction at the anode and avoid scale formation. While a better alloy for the anode does increase power delivered by the cell, it does not avoid the side reactions that cause problems and reduce the cell efficiency and output. Flushing fresh electrolyte through both anode and cathode compartments has been used in prior art galvanic cells to dislodge bubbles of hydrogen or oxygen gases and scale forming solids. This method temporarily improves cell performance but does not improve the capabilities of the cell. A porous cathode has been used to partition the hydrogen peroxide reactions from the anode compartment. Cell performance depends upon the levels of hydroxide ion in the electrolyte. High hydroxide ion concentrations have been used to improve the cathode while the corrosion at the anode is uncontrolled. At low hydroxide levels, aluminum hydroxide scale forms as a coating on the anode and other surfaces within the anode compartment. Formation of soluble aluminate ion depends upon sufficient levels of alkali hydroxide in the electrolyte.
Accordingly, there is a need in the art for an improved electrochemical cell utilizing an aluminum anode that greatly reduces electrode scaling and side reactions associated with present systems known in the art, thereby providing a high-energy galvanic cell.